Numerous animals, humans included, have adapted to living in
cold climates by increasing their body size. And now, researchers have
found that people living in colder regions have more obesity-related gut
microbes than their warm-region counterparts. Are bacteria the reason why it's harder to lose weight in the snow?

"Bergmann's rule" states that populations in
higher latitudes tend to be larger than those living in warmer latitudes. This
trend, which was first described by German biologist Carl Bergmann nearly two
centuries ago, typically applies to mammals and birds.

The rule also applies to
many human populations, such as the large-bodied Inuit, according to Taichi Suzuki,
an ecology and evolutionary biology PhD student at the University of California
Berkeley.

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Bergmann's rule is all about the ratio between body mass and
surface area. Larger animals tend to have greater body mass, and thus produce
more body heat. Compared with smaller animals, they also have a smaller surface
area relative to their body mass — this means, essentially, that they will
radiate less heat per unit of mass, helping them to stay warm in colder
climates. On the opposite side of the coin, animals that are relatively
small-bodied and live in a warm climate radiate more body heat per unit of
mass, allowing them to stay cool.

Over the last decade, we've discovered that the microbes that live in your guts can affect…
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So Suzuki wondered: Are these gut communities somehow
connected to the geographical trend in human body size?

Firmicutes vs. Bacteroidetes

To find out what role — if any — bacteria play in
the matter, Suzuki and his colleague Michael Worobey decided to look at how the
proportions of Bacteroidetes and
Firmicutes in your stomach changes, depending on your latitude.

The pair gathered data from six
studies, which contained gut microbial information on 23 populations (1020
people) from various latitudes, including places in South America, North
America, South Africa and Europe.

"Our major finding
is that in colder regions, at higher latitudes, you see more Firmicutes and
less Bacteroidetes," Suzuki says. People living at higher latitudes, such as in Europe, had a
significantly larger abundance of Firmicutes and smaller abundance of Bacteroidetes
than people living at lower latitudes. That is, so-called "healthy micrbiota" differ by geographical regions.

The abundances of Firmicutes (blue) increase at higher latitudes, while the abundances of Bacteroidetes (red) decrease at higher latitudes. Credit: Suzuki et al.

When Suzuki and Worobey teased apart the data,
they found that no single factor, including age, sex, or the studies' methods used
to detect the gut microbes, could explain the relative microbial abundance
pattern they found. For instance, men and women displayed similar relative abundances
of the two bacterial phyla. Likewise, the bacterial communities of children and
adults were similar. However, the abundances of elderly people ran counter to
the trend — the researchers note that the elderly (and infants) have a distinct
and less stable gut microbial communities than other people.

At this point, it's not clear what the trend means,
exactly. "It doesn't necessarily mean that microbes are causing the humans
in colder regions to get fat," Suzuki says. It could be the case, for example, that people in colder regions eat a more fatty diet, and this affects their gut microbial composition.

As part of his dissertation work, Suzuki conducted a similar
experiment in mice. "I knocked on people's doors and collected microbe samples from mice across North America," he explains. He found that
house mice in colder, higher-latitude regions had greater body mass and more obesity-associated
microbiota than mice living at lower latitudes.

Suzuki is now planning to do transplant studies with mice.
He will be taking the gut microbes from mice in colder and warmer regions and
putting them into lab mice, and then watching what happens to the mice's
weight. This approach will allow him to see if the microbes are causing the
change in body size, or if food is behind everything. Chances are, both factors play
a role, he says.

"My interest is
how microbes affect our health and evolution," Suzuki says. "People think obesity is a bad thing, but maybe
it's not a bad thing in terms of human evolution and natural populations. It
could be a good thing if you live in a cold place and need more fat to
survive."